In the field of lithographic printing plates, so-called presensitized (PS) plates comprising an aluminum support having provided thereon a light-sensitive layer composed of a light-sensitive composition have been employed. The aluminum support used in the presensitized lithographic printing plates generally has a surface roughened by a process selected from various roughening processes. The roughened surface is then etched with an acid or alkali aqueous solution, anodically oxidized and, if desired, subjected to surface treatment to render it hydrophilic. A light-sensitive layer is coated on the resulting support to obtain a presensitized lithographic printing plate, which is then exposed to light, developed, retouched, gummed, and the like to produce a printing plate.
Typically employed roughening processes include a mechanical roughening process, such as ball graining, wire graining, brush graining, liquid honing, etc.; an electrochemical roughening process called electrolytic graining; a chemical roughening process; and combinations of two or more of these processes. However, each of these techniques has respective disadvantages, as set forth below.
In the case of ball graining, there are problems in that high skills are required for selection of the composition (material) and the size of balls, control of water in carrying out abrasion, determination of abrasion time and evaluation of the finished surface from the nature of a batch system, and productivity is very inferior. In the case of wire graining, the roughness of the resulting surface of the aluminum plate is non-uniform. In the case of brush graining, high roughness is not obtained on the treated surface, and scattering is easily formed on the coarse face due to the wear of the abrasion brush used. Another disadvantage associated with wire graining is that the aluminum surface is scratched by the strong friction with the brush combined with the abrasive to form many sharp projections. These projections are likely to cause problems such as that the light-sensitive layer that should be removed by development remains to cause stains on the printing plate and that scratches are easily formed on the surface by rubbing of the treated surface (roughened surface) on handling of the aluminum plate.
In the case of liquid honing, since a slurry liquid having dispersed therein a fine abrasive powder is sprayed onto an aluminum sheet at a high rate by, e.g., compressed air, the fine abrasive powder easily sticks into the aluminum surface to form projections. Further, this technique cannot attain sufficient roughness because of weak impulsive force of the slurry liquid against the surface of aluminum. Significant wear of a jetting nozzle used for jetting the slurry liquid at a high rate is also a problem.
In the case of electrochemical roughening, achievement of a constant surface roughness requires precise control of electrolysis conditions, and the consumption of electric power is not small. Moreover, disposal of the waste liquor in which aluminum ions are accumulated entails high cost.
Finally the chemical roughening technique not only takes relatively much time for roughening, but also requires great expense for disposal of the waste liquor as in the case of the electrochemical technique, and is, therefore, not completely suitable for mass production.
In order to attempt to overcome some of the above-described disadvantages associated with each graining process, combined roughening processes have been described. One combination that has been described is that of brush graining or wire graining and electrolytic graining as disclosed in U.S. Pat. No. 2,344,510 and Japanese Patent Application (OPI) Nos. 123204/78, 145701/78, and 63902/79 (the term "OPI" as used herein means "unexamined published application"), and another is a combination of liquid honing and electrolytic graining as disclosed in Japanese Patent Application (OPI) No. 19593/85.
According to such combined processes, however, when brush graining is adopted as a first step, i.e., a mechanical graining step, stains are apt to be formed in the nonimage areas during printing; and when wire graining is adopted as the first step, the resulting printing plate has poor printing durability.
A system in which an abrasive slurry liquid is accelerated by a high-pressure liquid is suitable for mass production and excellent in stain resistance, but it is still unsatisfactory in providing sufficient printing durability, thus leaving room for further improvements to meet the market demand for further improved properties.
While a roughening process comprising forming a first roughness by liquid honing and then modifying the first roughness by brush graining has been proposed in U.S. patent application Ser. No. 886,625 now U.S. Pat. No. 4,714,528 (corresponding to Japanese Patent Application (OPI) No. 27192/87), a further improvement in printing durability has been strongly desired in the market.